Wheatstone SP-8, Technical Manual

Page: 125 / 128

Appendix Page - 4 SP-8 / Dec 95
A
PPENDIX
4) If you have checked external devices and connections, and feel that the
problem is within the console, double check all wiring before attempting to
troubleshoot the console itself.
Testing a "Live" (Powered-Up) Console – Precautions
(1) If a module must be removed, but remain connected while trouble–
shooting, place a piece of cardboard or other non-conducting material
across the console where the module will be placed. This will prevent
shorting, and also avoid scratching or marring the faceplates.
(2) Be extremely careful when using meter or oscilloscope test probes, to
avoid shorting a test point to an adjacent connection. This is especially
important when probing a pin 7 op-amp output, since the adjacent pin 8 is
at 18 volts.
(3) NEVER NEVER NEVER NEVER NEVER remove or insert an integrated circuit while the console is
powered up.
INTEGRATED CIRCUITS
The audio circuits of the console consist almost entirely of plug-in IC op-
amps. The types called out in the schematic drawings and parts lists are
chosen for optimum performance; in an emergency situation other types of
known matching pin-out and capability can be temporarily substituted. Some
useful troubleshooting hints for these circuits follow.
(1) Due to the large open-loop gain of the typical op-amp, the inverting
input of an amplifier, configured as an inverter with its non-inverting input
grounded, acts as a “virtual ground,” and signal levels at this point can be
expected to be extremely low. However, a circuit fault could result in a large
signal level at the inverting input, so it may be worth checking.
(2) When one of these ICs fails, it commonly swings its output to one of
the power supply rails. This should be a first check when a bad IC is
suspected. Measure the output pin of the IC directly (as opposed to
measuring after a coupling capacitor) under a no-signal condition and look
for a large DC offset at the output. Note SP-8 input, group and output
modules are power-protected from the main voltage distribution busses by
means of electronic fuses ("polyswitches") that automatically isolate the
module from the rest of the console in the event of a voltage failure. These
will reset once the fault is corrected and they have cooled off.
The SP-8 Control Room/Studio module does not have polyswitches; it is
protected by 3.3
Ω
fuse resistors which will generally become open circuits
when an IC fails, often with no visual indication. Whenever a fault is
suspected check the voltage on the module side of these resistors. If one
needs replacement be sure to stand it up from the circuit card as it can
become hot enough to burn the card under fault conditions. In a pinch any
low value 1/4 watt resistor can be used.
(3) The capacitive loading effect of a test probe may occasionally cause
oscillations in a high gain amplifier circuit. For this reason it is advisable,
when using meter probes to measure DC voltage in an amplifier circuit, to
isolate the “hot” lead from the circuit under test with a 10K resistor. This
A polyswitch is a device that
switches to a very high imped-
ance state when it's current
threshold is reached. The device
resets to a very low impedance
state when the fault condition is
cleared.
!